The invention relates to a process for the distillation of a mixture including 2,2′-diisocyanatodiphenylmethane, 2,4′-diisocyanatodiphenyl-methane and 4,4′-diisocyanatodiphenylmethane in order to isolate 4,4′-diisocyanato-diphenylmethane and mixtures of 4,4′- and 2,4′-diisocyanatodiphenylmethane containing little 2,2′-diisocyanatodiphenyl-methane.
Diisocyanatodiphenylmethane isomers are constituents of polyisocyanate mixtures of the diphenylmethane series, which occur on phosgenation of aniline/formaldehyde condensates, hereinafter also denoted polyaminopolyphenyl polymethanes.
The condensation of aniline and formaldehyde and the phosgenation of polyaminopolyphenyl polymethanes is known from the prior art. After the phosgenation of polyaminopolyphenyl polymethanes, phosgene is first completely removed. Then the higher homologues of diisocyanato-diphenylmethane (also denoted polyisocyanatopolyphenyl polymethanes) are separated. Pure 4,4′-diisocyanatodiphenylmethane is then separated from the remaining mixture of isomeric diisocyanatodiphenylmethanes, which mainly includes 2,2′-diisocyanatodiphenylmethane, 2,4′-diisocyanatodiphenyl-methane and 4,4′-diisocyanatodiphenylmethane. Various separation processes based on distillation or crystallization or a combination of distillation and crystallization are known in the prior art.
DE-A-2 322 574 may be mentioned as one example of the isolation of 4,4′-diisocyanatodiphenylmethane using a crystallization process. One disadvantage of the crystallization process is its elevated energy requirement because, especially if high purity 4,4′-diisocyanatodiphenylmethane is to be obtained, large quantities of refrigeration energy must be provided. DE-A-2 631 168 is an example of a distillation process for the separation of 4,4′-diisocyanatodiphenylmethane. The process describes the multistage working up of a mixture of polyisocyanatopolyphenyl polymethanes to yield diisocyanatodiphenylmethane isomers. After separation by distillation of the more highly functional isocyanates, i.e. those having more than 2 isocyanate groups per molecule, the first distillation stream occurring in this stage, which substantially contains 2,2′-diisocyanatodiphenylmethane (hereinafter abbreviated to 2,2′-MDI), 2,4′-diisocyanatodiphenylmethane (hereinafter abbreviated to 2,4′-MDI) and 4,4′-diisocyanatodiphenylmethane (hereinafter abbreviated to 4,4′-MDI), is introduced into a first column and separated into a further distillation stream and a bottoms stream. The bottoms stream may amount to up to 10% by weight of the first distillation stream. The second distillation stream is fractionated in a second column into an overhead stream, which contains highly volatile impurities, 2,2′-diisocyanatodiphenylmethane and 2,4′-diisocyanato-diphenylmethane, and a bottoms stream, which predominantly contains fractions of 2,4′-MDI and 4,4′-diisocyanatodiphenylmethane. This bottoms stream is separated in a third column into 4,4′-MDI and a distillate fraction enriched with 2,4′-MDI. In the final distillation stage, 4,4′-MDI with a content of less than 2% by weight 2,4′-MDI is distilled off.
DE-A-2 933 601 and DE-A-3 145 010, for example, describe further processes for the isolation by distillation of 4,4′-diisocyanatodiphenylmethane or of mixtures of 4,4′- and 2,4′-diisocyanatodiphenylmethane. DE-A-3 145 010 proposes initially stripping out 2,2′- and 2,4′-diisocyanatodiphenylmethane as the overhead product from the diisocyanatodiphenylmethane isomer mixture, while 4,4′-MDI, from which isomers have largely been removed, is obtained as the bottoms product. In a final distillation, any polymerization products which have formed during the exposure to elevated temperatures should be removed from this bottoms product, while the overhead product is subjected to further working up by distillation.
In conventional distillation columns, the feed stream is conventionally divided into two product streams: an overhead product and a bottoms product. A multicomponent stream is thus not completely fractionated. Any further separations which are required may, for example, be performed by subjecting either the bottoms stream or the overhead stream to another distillation step similar to the first. Further distillation steps may optionally be performed thereafter. In continuous processes, it is necessary in such cases to provide each distillation step with its own column together with the associated evaporators and condensers. Such a sequence of distillation steps thus entails not only considerable expenditure on plant and equipment but also a considerable energy input. The operating costs of such a multistage distillation process are correspondingly high. Furthermore, in the case of MDI distillation, a substantial residue is formed by the exposure to elevated temperatures over two or more distillation stages, this residue consisting of secondary products such as for example uretidiones and carbodiimides, so reducing the quantity of target product.